properly discover and use the homeserver client API url for the client

This makes the program actually work for cases, where the client API is
under a different subdomain (like "matrix.hamburg.ccc.de" with the user
id having "hamburg.ccc.de").

main.go

@@ -288,8 +288,15 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
+
+	clientWellKnown, err := mautrix.DiscoverClientAPI(context.Background(), homeserver)
+	if err != nil {
+		log.Fatal(err)
+	}
+	homeserverURL := clientWellKnown.Homeserver.BaseURL
+
 	client, err := mautrix.NewClient(
-		homeserver,
+		homeserverURL,
 		userId,
 		config.Token,
 	)